That is not necessarily so. From what I have read today, the emissions (NOX) are reduced with the use of urea, the VW software injects this from a reservoir bottle when it recognises the car is under test conditions. To be effective all the time, the bottle would have to be quite large and constantly topped up, which can be expensive. So VW cheated by having the software use it under test conditions only. The cars will only be producing the higher emissions when it needs the power, at other times like idling, slowing down or cruising, the exhaust gas recirculation system will allow a certain percentage of the exhaust gas to enter the combustion chamber and be burnt off a second time reducing the NOX. Engine manufacturers are constantly finding more ways to reduce emissions and make the engines smoother and more efficient all the time, through use of lower friction materials and auxiliary components.
As for other peoples comments on tax liability, that is all based on CO2 emissions not NOX emissions, CO2 emissions may well be unaffected by the use of the software so the liability may well be unaffected.